Method for protein isolation in anoxic conditions

ABSTRACT

The present invention relates to a method for the isolation of proteins that comprise disulfide-bonds in their native conformation. Essentially, a method of the present makes the use of reducing agents such as β-mercaptoethanol or dithiothreitol in protein isolation methods obsolete. A method of the present invention is particularly suitable for the isolation of precursor proteins such as proinsulin from recombinant cells.

FIELD OF THE INVENTION

The invention relates to a method for the isolation of proteins. Moreparticularly, the present invention provides methods for the isolationof proteins that comprise disulfide-bonds in their native conformation.

BACKGROUND OF THE INVENTION

The human peptide hormone insulin controls blood glucose levels duringfeeding and fasting and acts via cell surface receptors of liver andadipose tissue cells. Besides controlling uptake, storage and productionof glucose, insulin is also i.a. involved in the control of productionand breakdown of lipids.

Deficiencies in the supply of insulin result in elevated blood glucoseconcentrations (hyperglycaemia) and, in chronic form, are revealed bythe classic symptoms of diabetes mellitus (DM). Insulin is administereddaily to patients suffering from DM.

Mature human insulin is a peptide that is comprised of an A (alpha) anda B (beta) chain, linked by 2 inter-chain disulfide bridges. A thirddisulfide bridge connects two residues of the A chain. In proinsulin,the biosynthetic precursor, the A and B chains, are connected to eachother by the C peptide, which role is to aid in appropriate disulfidebridge formation between the A and B segments and to allow properfolding of the proinsulin molecule. In the last stage of maturation,proteolytic enzymes cleave at specific amino acid residues to releasethe C peptide thus forming the mature insulin.

Biosynthetic recombinant human insulin is presently i.a. manufactured asproinsulin-like polypeptides expressed in e.g. E. coli or yeast (seee.g. U.S. Pat. No. 5,593,860). In most cases, proinsulin is produced asa fusion protein or recombinant hybrid, wherein the proinsulin iscross-linked via methionine residues to a heteroprotein, such as forinstance human copper/zinc superoxide dismutase (hSOD). Normally, thesehybrids accumulate in the recombinant cells as intracellularprecipitated protein or inclusion bodies.

During manufacturing of recombinant proinsulin, the inclusion bodies,obtained by centrifugation after lysis of the cells, are washed with adetergent or a denaturant at a low concentration. Such treatment isrepeated to increase the purity of the desired protein. In order tominimize intermolecular hydrophobic interaction, and formation ofincorrect disulfide bonds, the washed inclusion bodies are dissolved ina denaturant, such as a urea or guanidine-HCl solution containing areducing agent such as dithiothreitol (DTT) or 2-mercaptoethanol, andrecovered by precipitation.

The hybrid is normally isolated and cleaved by cyanogen bromide (CNBr)in order to release the proinsulin polypeptide from the heteroprotein.The proinsulin is further modified by oxidative sulfitolysis toproinsulin S-sulfonate (See e.g. EP 0 055 945 and EP 0 196 056). Theproinsulin S-sulfonate is then further purified and refolded to a nativeconformation under reducing conditions by using reducing agents such asdithiothreitol (DTT), 2-mercaptoethanol, etc. or a redox system such asglutathione. Conversion of the proinsulin to insulin, i.e. removal ofthe C peptide, is achieved by the combined action of trypsin andcarboxypeptidase B. Finally insulin is purified through e.g.reverse-phase high performance liquid chromatography (RP-HPLC) andoptionally crystallized.

During the complete isolation procedure, from lysis of the recombinantcells through to proper folding of the proinsulin, free thiol groups ofthe cysteine residues comprised in the (fusion) protein, may formincorrect or aspecific intra- or intermolecular disulfide bridges. Thisresults in scrambled peptides and inactive hormones or in the formationof ‘aggregates’ of desired proteins and contaminating proteins (U.S.Pat. No. 6,150,134). Therefore, free thiol groups should either beblocked in order to prevent the formation of incorrect disulfide bridgesor procedures should involve selectively cleaving of incorrect disulfidebonds.

Disulfide bond cleaving may i.a. be achieved by: a) modifying thecysteine residues into cysteic acid by cysteic acid oxidation orperformic acid treatment; b) modifying the cysteine intoS-sulfo-cysteine by sulfitolysis (R—S—S—R→2R—SO_(S) ⁻); c) reduction bymeans of certain reducing agents, such as phosphines or mercaptans (seee.g. EP 0 379 162).

However, preventing the incorrect disulfide bonds to form is preferredover the use of “oxido-shuffling” agents following protein isolation,and to achieve this, again several reducing agents, such asdithiotreitol (DTT), β-mercaptoethanol, cysteine, glutathione,E-mercaptoethylamine or thioglycollic acid, are most commonly used.

The use of all above-mentioned reducing agents, however, poses problemsin that they pose a toxic risk, in that they are costly, and in thattheir contamination of the product requires additional purification.

Therefore, the conventional process for preparing recombinant proinsulinis proven to be less satisfactory since it involves complicated steps ofdissolution and sulfonation, purification, concentration, wherein therefolding of the proinsulin progresses inefficiently resulting inreduced yields of the desired protein.

Accordingly, there is a need for an improved process for the isolationof proteins which comprise disulfide-bonds in their native conformationin an efficient and less contaminating manner.

SUMMARY OF THE INVENTION

The present inventors have now found that the use of reducing agents inthe isolation of proteins comprising disulfide-bonds can be avoided orat least substantially reduced by performing said isolation under anessentially anoxic atmosphere.

The present invention provides a method for the isolation of proteinsthat comprise disulfide-bonds in their native conformation, said methodcomprising isolating said protein under essentially anoxic conditions.

By using the method of the present invention, disulfide-bonds comprisingproteins can now be extracted and isolated from the environment withoutthe need for the use of objectionable reducing agents. One of theadvantages of the method of the present invention is that no extractionof such reducing agents from the isolated and purified protein productis necessary any more and that thus the isolation procedure is moreefficient. Moreover, a method of the invention prevents the formation ofscrambled conformations of proteins and results in a high yield ofproteins in their stable, native conformation.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic flow chart of the major procedural steps comprisedin a method of the invention.

FIG. 2 illustrates the effect of sparging of 40 mL aliquots of isolatedinclusion body suspensions as described in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

The term “purification” or “purifying” and “isolation” and “isolating”as used herein is defined as the process of releasing and obtaining asingle constituent, such as a defined macromolecular species, from amixture of constituents, such as from a culture of recombinant cells. Inparticular, an isolation procedure herein involves the harvest and lysisof cells, the extraction of particular proteins from a crude cellextract obtained from said lysis and may further comprise the varioussteps of isolating subcellular structures such as inclusion bodies thatcomprise the desired proteins from other cellular components such asnucleic acids or polysaccharides. More in particular, a purificationprocedure herein involves the process of isolating a protein in a fairlypure form, i.e. freeing it from further impurities upon its isolation.

“Isolated” and “purified” refer to any molecule or compound that isseparated from its natural environment and is from about 60% to about99% free, preferably 80% to 99% free from other components with which itis naturally associated.

The term “recombinant” as used herein refers to a protein or nucleicacid construct, generated recombinantly or synthetically, e.g., in thecase of a protein, through the translation of the RNA transcript of aparticular vector- or plasmid-associated series of specified nucleicacid elements or of an expression cassette in a host cell. The term“recombinant” as used herein does not encompass the alteration of thecell or vector by naturally occurring events (e.g., spontaneousmutation, natural transformation/transduction/transposition) such asthose occurring without deliberate human intervention.

By “host cell” is meant a cell which contains a vector or expressioncassette and supports the replication and/or expression thereof. Hostcells may be prokaryotic cells such as E. coli, or eukaryotic cells suchas yeast, insect, amphibian, plant cells or mammalian cells. Preferably,host cells are bacterial or prokaryotic cells. A particularly preferredhost cell for the production of insulin is an E. coli host cellpreferably E. coli strain Sφ733.

As used herein, “vector” includes reference to a nucleic acid used intransfection of a host cell and into which can be inserted apolynucleotide. Vectors are often replicons.

The term “protein” or “proteins” as used herein refers to a polypeptideor any portion thereof.

The term “polypeptide” refers to a polymer of amino acids and does notrefer to a specific length of the product; thus, peptides,oligopeptides, and proteins are included within the definition ofpolypeptide. This term also does not refer to or exclude post-expressionmodifications of the polypeptide, for example, glycosylations,acetylations, phosphorylations and the like. Included within thedefinition are, for example, polypeptides containing one or more analogsof an amino acid (including, for example, unnatural amino acids, etc.),polypeptides with substituted linkages, as well as the modificationsknown in the art, both naturally occurring and non-naturally occurring.

The term “recombinant protein” as used herein refers to (1) apolypeptide of semisynthetic or synthetic origin resulting from theexpression of a combination of DNA molecules of different origin thatare joined using recombinant DNA technologies; (2) a polypeptide ofsemisynthetic or synthetic origin that, by virtue of its origin ormanipulation, is not associated with all or a portion of a protein withwhich it is associated in nature; (3) a polypeptide of semisynthetic orsynthetic origin that is linked to a polypeptide other than that towhich it is linked in nature; or (4) a polypeptide of semisynthetic orsynthetic origin that does not occur in nature.

The terms “inclusion body” and “refractile body” refer to theintracellular aggregates resulting from the non-specific precipitationof individual proteins. The formation of inclusion bodies and refractilebodies is a frequent consequence of high-level protein production in thecytoplasma. They are formed through accumulation of foldingintermediates rather than from the native or unfolded protein. Inclusionbodies or refractile bodies can be formed by any or all of the followingreasons: heterologous nature of expressed polypeptides; high proteinexpression rate; relatively high amount of hydrophobic protein thataggregates intermolecularly as a result of non-covalent association; andchaperones like helper proteins either being absent or inadequatelyavailable.

The term “disulfide-bond” or “disulfide-bonds” as used herein is definedas one, resp. more, cross-links between polypeptide chains or betweenparts of a polypeptide chain formed by the oxidation of cysteineresidues. The resulting disulfide is called a cystine.

The term “essentially anoxic” as used herein is defined as greatlydeficient or substantially lacking in oxygen and may range from areduction in the level of oxygen that is normally encountered in air, toa fully oxygenless condition of e.g. an atmosphere, a liquid environmentor a tissue.

“Protein conformation” refers to the characteristic 3-dimensional shapeof a protein, including the secondary (helices, sheet), supersecondary(motifs), tertiary (domains) and quaternary (multimeric proteins)structure of the peptide chain.

The term “native conformation” as used herein refers to thecharacteristic state, formation, shape or structure of a protein in thebiologically active form in a living system in which it is folded to aglobal minimum of Gibbs free energy as defined by C.B. Anfinsen (NobelLecture, Dec. 11, 1972).

“Refolding” refers to the in vitro process of transformation of aprotein after full denaturation by reductive cleavage of its disulfidebonds into a protein of native conformation.

Proteins with disulfide-bonds are generally not found in the cytoplasmwith the exception of the sulfhydryl oxidoreductases. However, proteinsthat are exported from the cytoplasm, such as for instance insulin andalkaline phosphatase, typically contain disulfide bonds. This is oftenattributed to the difference in reducing potential between the cytoplasmand the extracellular environment, whereby the presence of thioredoxinreductases in the cytoplasm is believed to constitute an importantfactor. Several studies have shown that both cellular activity andthioredoxin reductase activity is required to keep extracellularproteins, which would comprise oxidized disulfide bonds in their nativeconformation, in an inactive and reduced form within the cytoplasm(Derman et al., 1993. Science 262:1744-47; Derman and Beckwith. 1995. J.Bacteriol. 177:3764-70).

After lysis (or cell death or growth arrest due to substrate depletion)thioredoxine reductases are no longer capable of maintaining the cysteinresidues of proteins in reduced form and random folding of the proteinstarts with concomitant oxidation of cysteine residues into disulfidebonds. This process often results in aberrantly folded or scrambledproteins.

In order to obtain a pure extract of proteins in their nativeconformation which comprises the presence of disulfide bonds, such asfor instance insulin, from recombinant material, a key step is in theformation of correct disulfide bonds.

Conventional methods for the isolation of a protein that comprisesdisulfide bonds in its native conformation rely on the presence ofreducing agents. It has now been found that the application of anoxicconditions from the moment the reducing power of thioredoxine reductasesdiminishes until the moment that the protein can be stabilized orconditions for correct folding are provided, can effectively prevent theformation of scrambled proteins.

In a first aspect, the present invention provides a method for theisolation of proteins that comprise disulfide-bonds in their nativeconformation, comprising isolating said protein under an essentiallyanoxic atmosphere.

Using a method of the present invention will result in an isolatedprotein product that is essentially free of contaminating reducingagents as described above, and wherein a large and substantial part ofthe proteins are active and in the native conformational state andbiologically active.

Suitable proteins that can be used in a method of the invention areessentially proteins that comprise disulfide-bonds in their nativeconformation, but also other peptides may be isolated by a method of theinvention. Very suitable proteins are for instance peptide hormones suchas insulin, vasopressin, somatostatin, octreotide, endothelin I,knottin-like proteins, enzymes such as ribonuclease, epitopes such asepitopes of the Cn2 scorpion toxin, conotoxin, and/or LDL receptorepitope modules.

In general, disulfide-rich proteins, peptides or fragments thereof, beit either from viruses, bacteria, fungi (including yeast), plants,animals or humans, are valuable for studying structure-activityrelationships in e.g. drug design. Therefore, also to this field ofscience, the possibilities of the present invention to more efficientlyattain the native conformation of these proteins, has importantbenefits.

A method of the invention is suitable for the isolation of proteins fromany source that comprise disulfide bonds in the native conformation,such as associated with a virus or a prion or produced by a prokaryoticorganism, such as a bacterium, or produced by a eukaryotic organism,such as a yeast, a fungus, a plant, an animal, or a human cell.Preferably said protein is an extracellular protein that is in thereduced form or state when expressed in the cytosol of a cell of theproducing organism and that attains its native conformation when in theoxidized state.

The skilled person is capable of determining whether a proteincomprising disulfide bonds is (re)folded properly and in the nativeconformation. Such determinations may for instance comprise themeasurement of the properly folded, oxidized and digestedLys-Arg-Insulin intermediate, by HPLC analysis.

A method of the present invention is used preferably for the isolationof recombinantly produced proteins that comprise disulfide-bonds in thenative conformation. Recombinantly produced proteins can be eitherdirectly expressed or expressed as a fusion protein. Detection of theexpressed protein is achieved by methods known in the art such as, forinstance, radioimmunoassays, Western blotting techniques orimmunoprecipitation.

The manner of producing or (bio)synthesizing a (recombinant) protein isof no relevance to the method of the present invention. Recombinantproteins may in principle be prepared by any method known in the art,for example, by such methods as be used for the preparation ofrecombinant proteins, such as for example by recombinant yeast, morepreferably by a recombinant bacterium, most preferably by a recombinantE. coli cell, such as for instance described in U.S. Pat. No. 5,593,860.In many, but not necessarily all instances, such methods will result inthe formation of fusion proteins or hybrids. Therefore, a method of thepresent invention is very suitably used for the isolation of fusionproteins.

In another preferred embodiment, the method is used to isolate suchproteins as can be produced in inclusion bodies. In a most preferredembodiment, the method of the invention is used for the isolation ofinsulin precursor protein comprised in inclusion bodies as produced bythe recombinant E. coli strain Sφ733 carrying and expressing the plasmidpDBAST-RAT-N71 (Bio-Technology General Ltd., Rehovot, Israel), which isderived from the pDBAST-LAT vector (see U.S. Pat. No. 6,001,604 or WO96/20724).

The present method involves the performance of a conventional processfor the isolation of proteins, whereby this process is essentiallyperformed under anoxic conditions. In a method of the invention, theinitial steps of harvesting, lysing and protein extraction are performedunder anoxic conditions without the use of reducing agents describedabove.

A method of the invention comprises the isolation of a protein underessentially anoxic conditions. Preferably, anoxic conditions areattained by providing the isolation environment with an anoxicatmosphere, such as a nitrogen atmosphere, a carbon dioxide atmosphere,or an atmosphere of helium or another inert gas. Most preferably, anatmosphere of N₂ is used in a method of the invention to provide anoxicconditions during the isolation of proteins that comprise disulfidebonds in the native conformation.

Preferably, an anoxic condition as applied in a method of the presentinvention is a condition equal to an atmosphere containing between 0 and1 vol. %, preferably between 0 and 0.5 vol. %, even more preferablybetween 0 and 0.05% of O₂, or a liquid in equilibrium with saidatmosphere. In a most preferred embodiment, the anoxic condition is acondition equal to an atmosphere containing essentially 0 vol. % of O₂,or a liquid in equilibrium with said atmosphere.

In short, a method of the invention for isolating proteins that comprisedisulfide-bonds in their native conformation, does not comprise thegrowth of cells and/or the production of the proteins themselves.However, a method of the invention is performed on protein-producingmaterial such as production cells containing proteins, from the start ofharvesting of the cells, including the post-harvesting washing of thecells, through to the stabilization of the isolated proteins byfreezing, e.g. by freezing of the isolated inclusion bodies.

More in particular, when the reducing potential in the cytosol of thecell diminishes, as a result of the decreased activity of thethioredoxin reductases, e.g. when the cells stop growing, e.g. duringharvesting and post harvesting washings, the cells are preferablyalready brought under anoxic conditions. Anoxic conditions arepreferably maintained throughout the process until the start of therefolding of the protein.

A method of the invention may suitably comprise such steps as celllysis, for instance by enzymatic pre-treatment, followed by disruptionor fragmentation of the cells, e.g. by sonication, French pressure celltreatment or high shear mixing such as Ultra-Turrax® treatment, providedthat said lysis is performed under anoxic conditions. A further step ina method of the invention may comprise the separation or isolation ofthe inclusion bodies from the cytosol, in case the desired protein isproduced therein, under anoxic conditions. After this step it ispossible to store the isolated inclusion bodies and to stabilize theproteins comprised therein, for example by freezing, e.g. at −20° C., orcooling, e.g. at 2-8° C. Further process steps in the isolation andpurification of the protein are not necessarily performed under anoxicconditions.

After release from storage, or directly after the above isolation steps,the isolated and optionally further purified protein may be refolded tothe native conformation. Generally, for disulfide bonds containingproteins such refolding is performed in a dilute solution in order toavoid non-native intermolecular disulfide bridge formation. A veryappropriate method for refolding of insulin comprises the dissolution ofthe protein in a NaHCO₃ and EDTA containing buffer at pH 12.0, reducingthe pH to 11.2, treatment with charcoal and filtering. The polypeptidesare then folded by applying air to the solution to oxidize thedisulfides.

For fusion products, subsequent digestion of the fusion protein with anappropriate proteolytic enzyme may be performed to release the desiredrecombinant protein. The method of the invention may further comprisethe purification to substantial purity of proteins by standardtechniques well known in the art, including detergent solubilization,selective precipitation with such substances as ammonium sulphate,column chromatography, immunopurification methods, affinitychromatography, and others. See, for instance, R. Scopes, ProteinPurification: Principles and Practice, Springer-Verlag: New York (1982);Deutscher, Guide to Protein Purification, Academic Press (1990).

The invention will now be exemplified by the following, non-limitingexamples.

EXAMPLES

Following fermentation the culture liquid is brought to anoxiccircumstances by shutting the addition of pressurized air, removal ofoverpressure and flushing of the headspace above the culture and processliquids with nitrogen. Flushing of the headspace with nitrogen iscontinued during all subsequent processing steps, which include theharvest and washing of cells, cell lysis, and additional treatments ofthe inclusion bodies. Nitrogen flushing is also applied to processingequipment such as vessels, holding tanks, and auxiliary equipment suchas centrifuges, homogenizers, and filtration units. Nitrogen flushing isexecuted using fixed piping.

Example 1

Two identical 450 L scale cultures of the recombinant human insulinprecursor producing production strain Sφ733/pDBAST-RAT-N-7-1 were grownfrom contents of identical working cell bank ampoules. The course of thefermentations was followed and samples of both cultures were analysedusing validated analytical procedures. The results demonstrate that bothcultures proceeded in very comparable manner and that the obtainedabsorbance, dry weight, protein and relative insulin precursor finalconcentrations showed dissimilarities of 8% or less.

Following fermentation, both cultures were subjected to identicalinclusion body (IB) recovery procedures, which is schematically shown inthe flow chart of FIG. 1.

During the IB recovery procedure a blanket of nitrogen gas was appliedonly to the latter culture; the former culture was handled under aerobiccircumstances.

Following IB recovery the obtained inclusion body products weresubjected to small scale refolding and purification in order todetermine the amount of correctly folded insulin precursor protein usingHPLC.

The analysis demonstrated that per litre of aerobically processedculture, the yield of the correctly oxidized, folded, and digestedinsulin precursor amounted only 8.1% as compared to the yield of theculture processed anoxically.

Example 2

We were able to demonstrate the effects of different atmosphereconditions on the isolation of insulin peptide. This was done byexperiments in which an anoxically recovered suspension of inclusionbodies was dispensed into 3 aliquots of 40 mL. These aliquots weresparged for 16 h. using equal flow rates of nitrogen gas, pressurizedair, or oxygen gas, respectively. Next, the suspension and startingmaterial were subjected to small scale refolding and purification tomeasure the amount of insulin that was obtainable. The analyses showedthat sparging the inclusion body suspensions with pressurized air oroxygen gas resulted in sharply reduced yields of correctly foldedprotein, whereas sparging for 16 h. with nitrogen gas did not have suchan effect on the yield. This demonstrates that using the method of thepresent invention the yield of correctly folded protein can be increasedsignificantly when oxygen is excluded from the insulin precursorinclusion bodies during processing. The results of the experiment areshown in FIG. 2.

1. A method of isolating proteins that comprise disulfide-bonds in theirnative conformation, said method comprising isolating said protein underessentially anoxic conditions.
 2. The method according to claim 1,wherein said essentially anoxic conditions comprise an essentiallyanoxic atmosphere.
 3. The method according to claim 1, wherein saidprotein is a recombinant hybrid protein present as inclusion bodies ofrecombinant cells.
 4. The method according to claim 3, wherein saidisolation comprises the harvesting and disruption of said cells, theisolation of said inclusion bodies and/or stabilization of the isolatedproduct.
 5. The method according to claim 1, wherein said protein is aprecursor protein.
 6. The method according to claim 1, wherein saidprotein is insulin.
 7. The method according to claim 2, wherein saidessentially anoxic atmosphere is a nitrogen atmosphere.
 8. The methodaccording to claim 4, wherein said recombinant cells are cells of E.coli strain Sφ733 carrying and expressing the plasmid pDBAST-RAT-N-7-1.